Until now, scintillation detectors have been used for detecting gamma radiation and X-ray radiation, in particular in CT systems and dual-energy CT systems. In these detectors, the incident radiation is detected indirectly by the excitation of electrons and the conversion into photons. Moreover, direct-conversion detectors based on semiconducting materials, such as e.g. CdTe, CdTeSe, CdZnTe and CdZnTeSe, are known. These direct-conversion detectors are able to count individual photons and can thus directly detect the radiation.
Whilst some of the aforementioned semiconducting materials have been used successfully for X-ray radiation detection for a long time, the introduction of these materials into high-flux applications has not yet been successful. High-flux applications with a photon flux of more than 1*108 photons/cm2*s, as occur e.g. in CT systems, cannot yet be implemented thereby because in this case a particularly large number of charge carriers have to be separated very quickly and guided to the electrodes for detection. This is unproblematic for the negative electrons, but the less-mobile positive holes form a depletion zone, which adversely affects the electric field in the interior of the semiconductor material, i.e. it attenuates the field. The depletion zone basically follows the absorption profile of the X-ray radiation. It is therefore strongest on the side of the semiconductor detector facing the incident radiation. Depending on the size of the depletion zone, there can be a complete collapse of the electric field at this point. In the process, what holds true is that the stronger the depletion zone is, the more likely collapse of the electric field becomes. It follows that in order to ensure an even detector response that is independent of the photon flux, the formation of the depletion zone should be avoided or limited as much as possible.
First approaches for reducing the depletion zone in semiconducting detectors are already known. These consist of placing the entire detector at an angle with respect to the incident X-ray radiation. An article by P. M. Shikhaliev in Phys. Med. Biol. 51 (2006) describes the use of semiconducting detectors in a CT system, wherein these detectors are aligned obliquely to the incident X-ray radiation. This allows the use of thin semiconductor crystals with unchanging high photon absorption. In the process, there is a significant reduction in the polarization and an increase in the photon count rate.
Polarization is understood to be the reduction of the electric field by unmoving charges, generally bound to deep impurities, which can then capture the charge carriers generated by radiation, that is to say recombine with said charge carriers and thus suggest a significantly lower intensity of the radiation.
As a result of the trapped charge carriers, the effective mobility of the charge carriers is significantly reduced. However, a radiation detector must have a high charge-carrier mobility so that the electrons and holes generated during the irradiation can be separated in order thus to avoid the formation of a depletion in the detector and the effect of polarization caused thereby. According to this, the polarization limits the maximum flux that a direct-conversion detector is able to detect.
Moreover, a tilted detector allows higher spatial and energy resolutions, and shorter charge-carrier collection times, which allow high-flux applications in computed tomography. However, a simple tilt of the detector cannot realize a uniform angle of the surface with respect to the incident radiation because, due to beam widening effects, said radiation is incident on the surface at a different angle in the edge regions. Furthermore, this concept of the tilted detectors cannot be transferred to detectors for multi-row CT systems.